1. Technical Field
The present disclosure is directed to cable diagnostic test methods, systems and apparatus and, more particularly, to cable test methods, systems and apparatus that utilize “standing wave” principles to facilitate the identification and location of defect(s) along a power cable.
2. Background Art
With reference to FIG. 1, conventional shielded power cables generally consist of a conductor, normally fabricated from copper or aluminum, surrounded by a thin concentric semi-conducting screen, referred to as the conductor screen. The conductor screen, in turn, is surrounded by a concentric layer of insulation, the thickness of which increases with the voltage rating of the cable. This layer of insulation, in turn, is covered with a second thin concentric semi-conducting screen, referred to as the insulation screen. A concentric metal shield, in the form of concentric wires, overlapping metal tapes or other similar structure surrounds this screen. The entire assembly is housed in an insulating jacket which protects the cable against water ingress as well as physical and chemical damage. The cable conductor is typically maintained at an elevated voltage, while the outer metal shield is maintained at ground potential. Whereas the cable components may be based on different designs, modern cable insulation is generally of the following two types: (a) oil-impregnated paper or oil-impregnated polymer tape (laminated construction), and (b) extruded dry polymer. The cables are described as having a co-axial configuration.
As cables age in service, their insulation layers may develop weaknesses which pose a risk of precipitating a cable failure. As a result of aging, laminated insulation could become weaker over its entire length, but more often will develop discrete weaknesses as a result of water ingress, lack of sufficient oil or other structural and/or ambient conditions. Extruded polymer insulation is known to age in discrete locations due to voids, impurities, protrusions, water diffusion in the shape of trees (“water trees”) and other anomalies. Efforts have been made to address the properties and performance of polymer insulation compositions. See, e.g., U.S. Pat. No. 6,521,695 to Peruzzotti et al. Nonetheless, limitations associated with polymer insulations used in power cable manufacture, in conjunction with impurities or other conditions which force the electric stress to become concentrated, could produce carbonized defects in the shape of trees, called electrical trees, and eventually lead to early/undesirable cable failures.
Cable owners want to extend as much as possible the useful operating life of their cables while avoiding outages during normal service. The cables are, therefore, subjected to an initial commissioning test right after installation, and to periodic diagnostic tests (maintenance tests) during service to identify and correct any possible weaknesses. Excluding high-potential (“hipot”) withstand tests, diagnostic tests generally belong to one of two general categories: (a) global assessment of the insulating condition of the cable, and (b) assessment by partial discharge of discrete weaknesses. The following specific tests are commercially available under each category:
(a) Global Condition Assessment
Global condition assessment tests are generally designed to assess overall deterioration of certain insulating (dielectric) properties of the cable. Three specific test methods are noted:                Measurement of the dissipation factor of the overall cable when subjected to various voltage levels at one fixed frequency (such as 50/60 Hz, or 0.1 Hz). The dissipation factor is often referred to as tangent delta (the trigonometric tangent of the angle by which the total current drawn by the cable differs from that drawn by an equivalent ideal capacitor without loss). The tangent delta (tan δ) is a measure of the dielectric losses in the cable.        Measurement of the global dissipation factor and dielectric constant of the entire cable as a function of various frequencies while the voltage may assume several different levels. This method is also referred to as dielectric spectroscopy.        Measurement of the time it takes for a cable to recover its voltage after it has been charged to a certain voltage level with a direct current (dc) and momentarily shorted or, alternatively, the magnitude of the recovered voltage in a given time. Another dual method is the measurement of current as a function of time after having permanently shorted the cable. The first method is often referred to as “the return voltage” method, and the second as the “relaxation current” method. Both methods are based on dielectric polarization/relaxation principles.        
(b) Partial Discharge Measurement
Discrete defects often emit a very small electrical signal of very short duration (a partial discharge) when the cable is subjected to a voltage stress exceeding a threshold, or inception, level. Like radar technology, the site of a partial discharge can be accurately located by means of methods based on traveling electromagnetic waves and their reflections.
Additional prior art techniques for detecting faults and defects in electrical cables are described in the patent literature. U.S. Pat. No. 4,887,041 to Mashikian et al. describes a method and apparatus for detecting the locations of incipient faults in an insulated power line. In an exemplary embodiment of the Mashikian '041 patent, the method involves opening one end of the power line (if it is not suitably terminated to reflect high frequency pulses), applying an excitation voltage to the other end of the power line at an excitation point, detecting a first high frequency pulse produced by a discharge in the power line and transmitted on the power line to the excitation point, detecting a first reflection of the pulse from the open end of the power line to the point of excitation, detecting the travel time of a reflection of the first pulse from the excitation point to the open end of the power line and return to the excitation point, and dividing the time between the detection of the first pulse and the first reflected pulse by the detected travel time. The Mashikian '041 patent further discloses methods that detect discharge pulses which occur in a predetermined range of magnitude of the excitation voltage and discharge pulses which reside within predetermined ranges of magnitudes. The discharge sites may be detected using either reflected voltage pulses or reflected current pulses.
A further prior art teaching is provided in U.S. Pat. No. 5,272,439 to Mashikian et al. The Mashikian '439 patent discloses a method and apparatus for locating an incipient fault at a point along the length of an insulated power line that, in exemplary embodiments, involves the application of an excitation voltage at an open end of the power line. The signal pulse transmitted along the power line to the open end is passed through a high pass filter to remove the portion of the signal which is at a frequency below the excitation voltage and its harmonics. The filtered signal is amplified and passed through a band pass filter to remove a high frequency portion of the signal containing a large proportion of noise relative to the frequency of the partial discharge frequency from the incipient fault. This filtered signal is passed to a digital storage device adapted to be triggered by a signal of a predetermined amplitude, and the triggered digital storage device receives the amplified signal directly from the amplifier and stores digital data concerning amplitude and time for the peaks of the amplified signal for a predetermined period of time. The stored digital data is processed to identify the peaks reflecting the point of partial discharge in the power line.
Practical adaptations of the foregoing partial discharge location methods and their usefulness in performing diagnostic tests on installed cables are described in several technical publications, a recent typical example of which is an article published in the July/August 2006 issue of the IEEE Electrical Insulation Magazine, Vol. 22, No. 4, entitled “Medium-Voltage Cable Defects Revealed by Off-Line Partial Discharge Testing at Power Frequency,” by M. Mashikian and A. Szatkowski.
Partial discharge diagnostic methods are generally effective in finding cavities, indentations made with tools, screen or shield protrusions, and electrical trees in cables with extruded polymer insulation. Such methods are also generally effective in locating defective areas in oil-impregnated laminated insulation due to such causes as lack of oil, embrittled paper with carbonized tree-like formation (commonly referred to as tracking) and/or water globules. However, partial discharge techniques are not effective in directly identifying the location of water trees in extruded polymer insulation, nor are such methods effective in identifying/locating non-condensed moisture in oil-impregnated laminates. In that regard, it is noted that moisture—without the existence of condensed water could constitute an important cause of failure in oil-impregnated paper insulated cables. Moisture, water and water trees lead to a localized increase in the dissipation factor and, possibly, the dielectric constant of the insulation. Despite the significance of these factors in cable performance, a global condition assessment test may be unable to detect this condition without ambiguity, at least in part because such methods provide average values covering the entire cable length. None of the global condition assessment methods can localize such discrete defects and, unless discrete defects are located with precision, the diagnostic method loses its attractive economic advantage and overall value/reliability.
Accordingly, a need remains for methods, systems and apparatus for detecting and locating with precision defects and potential defects in power cables. These and other needs are satisfied by the methods, systems and apparatus disclosed herein.